Description
RLC Non-Transparent Mode (NT-RLC) is one of the three operational modes of the Radio Link Control layer in GSM and EDGE radio access networks, as standardized in 3GPP TS 44.060. The RLC layer sits above the Medium Access Control (MAC) layer and is responsible for the segmentation, reassembly, and in-sequence delivery of data units, as well as error correction through retransmission. NT-RLC is specifically used for packet data services, most notably for GPRS and EDGE.
In NT-RLC, the protocol operates in Acknowledged Mode (AM). This means it implements an Automatic Repeat Request (ARQ) mechanism to ensure reliable data delivery. When a transmitting entity (like the BSS) sends a block of data, it waits for an acknowledgement (ACK) from the receiver. If an ACK is not received within a timeout period, or if a negative acknowledgement (NACK) is received, the block is retransmitted. This process continues until successful delivery or until a maximum retransmission limit is reached. The RLC header contains sequence numbers, control bits for polling, and status information to manage this ARQ process.
The architecture involves RLC entities at both the Mobile Station (MS) and the Base Station Subsystem (BSS). These entities manage transmit and receive windows, maintain state variables for sequence numbers, and handle the creation and interpretation of RLC/MAC block headers. NT-RLC works in conjunction with the Logical Link Control (LLC) layer above it, which provides end-to-end reliability between the MS and the SGSN. NT-RLC's reliability is thus hop-by-hop (over the air interface), while LLC provides additional end-to-end security and acknowledgment.
Its role is to provide a reliable radio bearer for packet-switched data services over the Um interface. It masks the errors inherent in the wireless medium from the higher layers, ensuring that data packets are delivered correctly and in order across the air link. This is crucial for applications like email, web browsing, and file transfer over GPRS/EDGE, where data integrity is important but strict real-time delay constraints are not. It represents a more complex but robust alternative to Transparent Mode (used for voice) and Unacknowledged Mode.
Purpose & Motivation
NT-RLC was developed to support the introduction of packet-switched data services in GSM networks, namely General Packet Radio Service (GPRS) and later Enhanced Data rates for GSM Evolution (EDGE). Prior to GPRS, GSM primarily supported circuit-switched voice and low-rate data using Transparent Mode RLC, which offered forward error correction but no retransmissions, making it unsuitable for reliable data packet transfer where bit errors could corrupt entire data blocks.
The problem it solved was how to provide a sufficiently reliable data link over the error-prone wireless channel for non-real-time applications. Transparent Mode was inadequate for data as it could not recover from lost or corrupted frames. NT-RLC, with its ARQ mechanism, was created to fill this gap. It was motivated by the need to enable efficient IP-based services on mobile networks, requiring a data link protocol that could guarantee delivery without the permanent resource allocation of a circuit-switched connection.
It addressed the limitations of the existing modes by introducing a selective retransmission capability. This allowed the network to trade off increased latency (due to retransmissions) for dramatically improved reliability, which was acceptable for early mobile data applications. NT-RLC became the foundation for the bursty, efficient packet data transmission that defined GPRS and EDGE, enabling the first widespread mobile internet experiences.
Key Features
- Operates in RLC Acknowledged Mode (AM) with ARQ
- Provides reliable data transfer through selective retransmission
- Uses sequence numbering for in-sequence delivery and duplicate detection
- Includes polling mechanism for receiver status reports
- Manages transmit and receive windows for flow control
- Works in tandem with LLC layer for end-to-end reliability
Evolution Across Releases
NT-RLC was already a mature feature from earlier GSM/GPRS specifications. In the context of 3GPP Release 8, it was maintained and referenced as part of the continued support for GERAN (GSM/EDGE Radio Access Network) within the EPS architecture. The specifications were consolidated, but the core NT-RLC protocol mechanics remained unchanged from its original GPRS design.
Defining Specifications
| Specification | Title |
|---|---|
| TS 44.160 | 3GPP TR 44.160 |